### Home > PC > Chapter 7 > Lesson 7.2.4 > Problem7-80

7-80.

Write the equation of the circle described.

1. Center at the origin, containing the point $\left(−6, −8\right)$.

Use the general form for a circle to find the radius.
$x^{2} + y^{2} = r^{2}$

2. Center $\left(7, 5\right)$, containing the point $\left(3, −2\right)$.

Use the form below to find the radius.
$\left(x − h\right)^{2} + \left(y − k\right)^{2} = r^{2}$

$\begin{array}{l}\left(x − 7\right)^{2} + \left(y − 5\right)^{2} = r^{2}\\\left(3 − 7\right)^{2} + \left(−2 − 5\right)^{2} = r^{2}\\16 + 49 = r^{2}\\65 = r^{2}\end{array}$

$\left(x − 7\right)^{2} + \left(y − 5\right)^{2} = 65$